The present invention relates generally to a system and method for automatically running a vehicle at a desired cruising speed.
Japanese Patent Application First Publications Sho 47-35692, Sho 56-99518, Sho 60-71341, and Sho 60-50031 and Japanese Patent, Application Second Publication Sho 53-7592 exemplify such vehicle cruising speed running systems. In such systems as disclosed in the Japanese Patent Application Publications, a set switch is provided for setting a vehicle speed to a desired cruising speed when operated to turn on. When the set switch is operated, a vehicle speed adjusting mechanism (a throttle actuator) associated with a throttle valve of an engine mounted in the vehicle and independent of an accelerator pedal is actuated to adjust an angular displacement (opening angle) of the throttle valve so that the vehicle speed matches with the set cruising speed. In addition when a brake pedal or clutch pedal is operated during such a cruising speed control operation as described above, such a control operation as to maintain the vehicle speed at the set cruising speed is released and the vehicle speed is thereafter reduced. On the other hand, such a system as described above includes a resume switch. When the resume switch is operated, the system automatically returns the currently reducing vehicle speed due to the release of the cruising control operation to the set cruising speed before the operation of such a pedal as described above (resume function) at a constant acceleration. Furthermore, an acceleration switch is provided for running the vehicle at a constant acceleration when operated to turned on. When the acceleration switch is turned on, the control for the constant acceleration of the vehicle is started in which the opening angle of the throttle valve is adjusted so that the actual acceleration of the vehicle accords with the set acceleration. Thereafter, when the acceleration switch is operated turn off, the vehicle cruising speed control is started in which the vehicle speed is adjusted to reach a target vehicle speed which is the vehicle speed at the time of off operation of the acceleration switch. When the actual acceleration exceeds the set acceleration during the transfer of control from the constant acceleration to the cruising speed, a gain of vehicle speed control is corrected so as to prevent an overshoot of the vehicle speed.
From among these Japanese Patent Application Publications, for example, in the Japanese Patent Application First Publication (Tokkai) Sho No. 47-35692 published on Nov. 25, 1972, the automatic vehicle cruising speed running system for the vehicle calculates an integration error .epsilon..sub.I between a desired target vehicle speed V.sub.s and actual vehicle speed V for each predetermined period and calculates an proportional error .epsilon..sub.P between the present actual vehicle speed V(t) and vehicle speed V(t-.DELTA.t) before one period of the predetermined period in the following equations (a) and (b). Thereafter, a rate of change .DELTA..theta. in the angular displacement of the throttle valve is calculated from the following equation (c) using the integration and proportional errors .epsilon..sub.I and .epsilon..sub.p and gain constants K.sub.I, K.sub.p. EQU .epsilon..sub.I =V.sub.S -V (a) EQU .epsilon..sub.P =V(t-.DELTA.t)-V(t) (b) EQU .DELTA..theta.=K.sub.I .times..epsilon..sub.I +K.sub.P .times..epsilon..sub.P (c)
In the system disclosed in the above-identified Japanese Patent Application document, the vehicle speed is feedback controlled, a total weight of the vehicle is changed due to the change in the number of occupants, and a gradient of road on which the vehicle runs is unknown.
Therefore, especially in a case when the vehicle runs on a rugged road, it is very difficult to always control the vehicle speed through the opening angle of the throttle valve and transmission shift position. Hence, the vehicle occupant(s) feels unpleasantly with the opening angle of the throttle valve varied unregularly and a hunting occurs in the vicinity of the target vehicle running speed so that it becomes, at this time, difficult to maintain the vehicle speed at the cruising speed.
In addition, in the Japanese Patent Application First Publication (Tokkai) Sho No. 56-99518 published on Aug. 10, 1981, with the resume switch operated so as to return the vehicle speed to the set vehicle speed V.sub.m, the system calculates the vehicle acceleration for each predetermined period (. e.g., 0.3 seconds) when the actual vehicle speed V is below a vicinity of the desired set cruising speed (V&lt;V.sub.m -.DELTA.V, wherein .DELTA.V=3 km/h). Then, the constant acceleration control is carried out such that the vehicle acceleration indicates a constant (for example, 2 km/h.sec) and, on the other hand, the control of the vehicle speed is transferred to the normal cruising speed control when the actual vehicle speed is above the vicinity of the set cruising speed (V.gtoreq.V.sub.m -.DELTA.V).
However, since a feedback control mode is used to carry out the constant acceleration in the vehicle cruising speed running system disclosed in the above-identified Japanese Patent Application Publication (Sho No. 56-99518), it is difficult to maintain the acceleration level constant in a case where a large disturbance such as a gradient resistance is added to a normal running resistance, when the vehicle runs on a rugged road. In this case, the opening angle of the throttle valve is unnecessarily varied.
In the Japanese Patent Application First Publication (Tokkai) Sho No. 60-50031 published on Mar. 19, 1985, the throttle valve is once fully opened for a predetermined interval of time when the cruising speed control is started in response to the on operation of the set switch to prevent a temporary reduction of the vehicle speed and the predetermined interval of time is extended in response to the change in the vehicle speed during the start of cruising speed control. To derive the extended predetermined interval of time, it is necessary to accurately detect the change in the vehicle speed. Hence, with the detection error of a vehicle speed sensor for sensing the vehicle speed taken into account, the extended interval of time is set longer to some degree. However, in this case, since the throttled valve is fully opened for the above-described extended interval of time set longer to some degree when the vehicle cruising speed control is started during the vehicle run on a descending slope such that the vehicle speed can be maintained at the target cruising speed with the throttle valve fully closed, the vehicle speed exceeds the target cruising speed and then the throttle valve is immediately closed. Thereafter, the vehicle speed is remarkably varied so that the vehicle occupant(s) feels unpleasant.
Furthermore, the vehicle cruising speed running system in the Japanese Patent Application First Publication (Tokkai) Sho No. 60-71341 has also a problem described below. That is to say, it is difficult to carry out an accurate control of constant acceleration of the vehicle in a case when the constant acceleration control of the vehicle is carried out during the vehicle run on the rugged road, since the control over the vehicle speed lags with respect to a remarkable change in the vehicle speed caused by the gradient of road. In addition, when the vehicle runs on the rugged road with the acceleration switch turned off after the once on operation thereof so that the vehicle speed control is transferred from the constant acceleration control mode to the cruising speed control mode, an overshooting and/or undershooting of the vehicle speed occur even if corrections of the feedback control and vehicle speed control gain are carried out. Consequently, the vehicle speed is remarkably varied with respect to the target cruising speed.